A chirped signal generally refers to a sinusoidal signal that varies in frequency over time, typically sweeping back and forth over a predetermined frequency range at a predetermined periodicity. Chirped signals are used, for instance, in radar applications. For instance, in radar, it is often desirable to generate a chirped transmit signal so that the delay between transmission of a wave and the return of the reflection of the wave off of a tracked object can be determined by measuring the frequency of a received reflected signal relative to the frequency of the contemporaneous transmitted signal.
In at least one common form of radar, the same antenna is used to transmit an outgoing signal (waves) and to receive the resultant return signals reflected from objects in the path of the transmitted waves. Particularly, the transmitter is turned on for a specified period to feed a transmit signal to the antenna, then it is turned off and the receiver is turned on to receive any reflected signals and then the receiver is turned off. The process is repeated continuously.
In other forms of radar, separate antennas are used for transmit and receive functions so that each transmitter circuit, receiver circuit, and associated antenna continuously transmits or receives. The separate transmit and receive antennas may be physically close to each other or physically remote from each other. However, if they are close to each other, the signal transmitted from the transmit antenna will likely couple onto the receive antenna. This is generally undesirable and requires equalization of the receive signal in order to compensate for such interference.
With respect to radar applications in which high accuracy is required and/or a large geographic area is to be observed, such as military and avionic applications, it is common to use antenna arrays comprising a plurality of antennas physically spaced from each other over a designated geographic area. In such antenna arrays, each transmit antenna transmits a signal. All transmit antennas transmit the same signal, except that the phases may be made different for purposes of beam forming, as is known in the art. Particularly, it is known in the art to send out a highly directional transmit signal by adjusting the phases of the transmit signals provided to each transmit antenna relative to each other to beam from the collective transmit signal to create a highly directional beam. It is common to sweep that signal over an angular range (e.g., to 60° on either side of a forward-looking direction) using this type of beam forming. The various antenna elements in the antenna array send out a collective wave front that reflects off of objects in its path and returns to the receive antennas.
Each receive antenna, of course, receives whatever waves pass by that antenna, which is likely to include (1) waves that are reflections of waves generated by the transmit antennas (which, as noted above, may be the same antennas as the receiving antennas), (2) waves received directly from other transmit antennas in the antenna array, and (3) waves from other sources of interference, such as other electronic equipment in the area. Only the first signal is desired. The rest essentially are interference.
In virtually any type of communication or telemetry system (such as radar), a received signal may need to be equalized in order to compensate for various forms of interference, echo, channel impulse response, and other unwanted signal components. It therefore may be necessary to equalize the signal received at a receive antenna in an antenna array of a radar system to compensate for (e.g., cancel) the signals that are the transmit signals from all of the transmit antennas in the array.
Equalization in digital communication or telemetry systems often is performed in the digital domain, for instance, by finite impulse response (FIR) filters. If the unwanted signal component has a wide bandwidth, such as would be the case for a chirped radar transmit signal, which might be swept over a very broad bandwidth, the necessary FIR filter to cancel such a signal component will need to have many taps and be extremely long and complex.
Another form of unwanted signal component is echo within a channel. Generally, any impedance mismatch in a signal path will cause a part of the signal to reflect back toward the transmitter. Impedance mismatches typically exist at the interface between two separate physical components in a signal path. Thus, for instance, in a radar system, there may be an impedance mismatch at the interface between a connector and a wire, between any two pieces of equipment, and even at the interface between the metal surface of the antenna and the air. The reflected signal returns to the transmitter after the propagation delay of the path from the transmitter to the reflecting interface and back and gets mixed in with the transmitted signal. A received signal also may require equalization in order to cancel this type of echo interference.
For purposes of clarity, this latter type of return signal shall be referred to as an “echo”, while the signals that are reflected off of objects within the range of the radar in a radar system are called “reflected” signals in order to clearly differentiate between the two different types of reflected signals.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for equalizing a chirped signal.